Tropical Rainfall Intensifies While the Doldrums Narrow

This satellite image shows a textbook Intertropical Convergence Zone (ITCZ) on 24 November 2010 that stretches all the way across the Pacific Ocean. New research improves understanding of ITCZ and shows that rainfall in the zone is intensifying. Credit: NASA GOES Project Science

Silence is not golden around Earth’s equator—when trade winds settle down, it spells disaster for sailing ships. In the Northern Hemisphere, trade winds blow southwest toward the equator, whereas Southern Hemisphere trade winds blow northwest. These winds meet at the Intertropical Convergence Zone (ITCZ), a ring around Earth traditionally known as the doldrums for its periodically calm winds that once trapped sail-driven seafarers.

ITCZ is a major tropical atmospheric feature. In this zone, hot air rises through the troposphere as it follows the Hadley cell, causing frequent thunderstorms and heavy rainfall. Seasonal shifts of ITCZ toward and away from the equator give rise to the intensely wet and dry seasons experienced by residents of the tropics. Forecasts of future global weather patterns depend heavily on accurately modeling the characteristics of ITCZ, but scientists have struggled to objectively identify ITCZ using existing measurements. Now Wodzicki and Rapp have improved upon previous efforts to identify ITCZ’s central location, its northern and southern boundaries, and the monthly intensity of the rainfall it causes.

The scientists modified an algorithm to identify and characterize ITCZ. They applied it to satellite observations from the NASA Tropical Rainfall Measuring Mission’s Microwave Imager (TMI), rainfall data from NASA’s Global Precipitation Climatology Project (GPCP), and the European Centre for Medium-Range Weather Forecasts’s ERA-Interim data set, which provides detailed climate data for every year since 1979.

The ERA-Interim data place the center of the Pacific ITCZ at approximately 8°N. Although the center has remained relatively constant over the past 36 years, TMI and GPCP data show that the southern and northern boundaries have shifted toward the center, effectively narrowing the ITCZ band in the Pacific. ITCZ’s boundaries vary, but its width was more variable prior to 1998, when a large El Niño event occurred and the Pacific Decadal Oscillation changed phase.

The data also show that rainfall in ITCZ has intensified, especially toward the center of the zone. Further research is needed to pinpoint what drives ITCZ’s shifting characteristics and whether the increased rainfall in ITCZ is linked to its narrowing width. Meanwhile, the authors’ ITCZ algorithm could provide a quantitative benchmark for how well ITCZ characteristics are represented in our climate models. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2015JD024458, 2016)

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